1. Field of the Invention
The present invention relates to a method of warming up a fuel cell system including a fuel cell, a heater for heating the fuel cell, and a capacitor electrically connected to the fuel cell. The fuel cell includes an electrolyte electrode assembly interposed between a pair of separators. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes.
2. Description of the Related Art
Generally, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between separators. The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
If the fuel cell has a low temperature at the time of starting operation, power generation can not be performed efficiently. It takes considerable time to raise the temperature of the fuel cell to a desired temperature for power generation. In particular, if operation of the fuel cell is started at a temperature below zero (freezing temperature), water condensation is likely to occur due to the heat radiated outwardly from the fuel cell, and the water produced in the reaction of the fuel cell is not smoothly discharged from the fuel cell. Thus, the desired power generation performance of the fuel cell may not be achieved.
In an attempt to address the problem, the U.S. Pat. No. 5,798,186 discloses a fuel cell system in which a fuel cell stack is connected to an external electrical circuit, and the supply of electric current to the external electrical circuit from the fuel cell stack is commenced such that the temperature of the membrane electrode assembly exceeds the freezing temperature of water.
In the fuel cell system of the U.S. patent, the temperature of the entire fuel cell stack is raised by self-heating. If operation of the fuel cell stack is started at a low temperature, a large amount of heat energy is needed for warming up the entire fuel cell stack. If an electrical heater is used for warming up the fuel cell stack, a considerably long time is needed, and the electrical heater needs to have a considerably large electric capacity. In particular, if operation of the fuel cell stack is started at a temperature below the freezing temperature, the water produced in the fuel cell stack may be frozen undesirably in the gas diffusion layers or reactant gas passages, and the warming up operation may not be carried out continuously.